The present invention relates to an improved resin system for imparting wet strength to paper which yields a sheet which is readily repulpable.
Paper which is manufactured with chemical additives to improve the physical properties when in contact with a water-containing medium is known as wet strength paper. The wet strength resins which provide these properties are classified as being either "permanent" or "temporary," based on the permanence of the wet strength they provide. A number of chemical treatments have been used to impart permanent wet strength to paper, but the most common, by far, is the use of aminoplast and polyamide-epichlorohydrin (PAE) resins.
As a result of the heightened awareness and increased demand for paper products containing recovered cellulose fiber, efforts have been undertaken to develop paper products which are more readily recyclable. Commercially available wet strength paperboard products are difficult to repulp because they utilize permanent wet strength resins. The addition of permanent wet strength resins interferes with and detracts from the repulpability of paper. Wet strength paper generally cannot be defibered and repulped in neutral water without extraordinary means. Permanent resins are resistant to hydrolysis and retain their properties during repulping. The mechanism by which they provide wet strength is through bonding to or encapsulation of the cellulose fibers to provide a water-resistant, hydrolytically-stable, polymer-reinforced cellulose fiber network. Paperboard treated with aminoplast resin requires high temperatures and/or low pH during repulping to be recycled. On the other hand, high pH and elevated temperatures are required to repulp PAE-treated papers. Polyamide and polyamine-epichlorohydrin (PAE) resins form ether linkages with the hydroxyl groups in the cellulose through an epoxide linkage. These bonds are difficult to break.
Temporary wet strength resins are generally distinguished from permanent wet strength resins in that wet strength achieved using temporary wet strength resins is essentially lost after 10 to 30 minutes soaking in water at neutral pH and room temperature. Temporary wet strength resins are well known and have been used in the art to make disposable products, such as toweling and tissues. Temporary wet strength agents are generally hydrolytically unstable or shear sensitive. These properties enable the resin to break down readily when the product is commercially repulped. While temporary wet strength agents are more compatible with repulping than permanent wet strength agents, they do not develop satisfactory wet strength for most paperboard applications.